I don't know if this would be a good alternative suggestion but..
we have orion>preferences>websites>pagezoom so I hypothesize that the address bar would only display the zoom level if it does not match anything set into the websites zoom preference list?
this is hopefully a good example;
in preferences - set slashdot to use 120%
visit slashdot at default 100% zoom and it shows up in address bar
set zoom to 120% and the address bar value disappears since it matches the preferences
I know this suggestion won't cover all usercases but still